John Chrysostom

But why did he go on baptizing now? Because, had he left off, it might have been attributed to envy or anger: whereas, continuing to baptize, he got no glory for himself, but sent hearers to Christ. And he was better able to do this service, than were Christ's own disciples; his testimony being so free from suspicion, and his reputation with the people so much higher than theirs. He therefore continued to baptize, that he might not increase the envy felt by his disciples against our Lord's baptism. …

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Therefore Christ especially began to preach at the time when John was imprisoned, on account of the jealousy of the Baptist's disciples. I think that John's death was also permitted to happen very soon so that all the devotion of the people would transfer to Christ, and the people would not be divided in their thoughts about both John and Christ.

Commentary on John ↗

Bede

He evidently here is relating what Christ did before John's imprisonment; a part which has been passed over by the rest, who commence after John's imprisonment.

Catena Aurea by Aquinas ↗

Thomas Aquinas

The time is indicated when he says, John had not yet been thrown into prison. He says this so that we may know that he began his narrative of Christ's life before the other Evangelists. For the others began their account only from the time of John's imprisonment. So Matthew (4:12) says: "When Jesus heard that John had been arrested, he withdrew into Galilee." And so, because they had passed over the things that Christ did before John's imprisonment, John, who was the last to write a Gospel, supplied these omissions. …

Commentary on John ↗

Bonaventure

"For John had not yet been cast into prison," because then his baptism ceased, the master being absent, for it was temporal. Concerning the imprisonment of John, Matthew 14: "Herod seized John and bound him and cast him into prison."

Commentary on John, Chapter 3 ↗